1. Field of the Invention
The present invention relates to a driving apparatus for negative-impedance driving a load and, more particularly, to a driving apparatus comprising a temperature compensation circuit which performs temperature compensation for preventing or eliminating a change in drive state caused by a change in temperature of a load and prevents the load from being heated beyond a safety range during temperature compensation.
2. Description of the Prior Art
In general, an electro-magnetic transducer (a dynamic electro-acoustic transducer) such as a speaker obtains a drive force by supplying a current i through a coil (copper wire coil) in a magnetic gap of a magnetic circuit. The drive force F appearing in the copper wire coil is given by: EQU F=B.multidot.l.multidot.i
where l is the conductor length of the coil, and B is the strength of a magnetic field of the magnetic gap. However, since the coil formed of a copper wire or the like has a positive temperature coefficient, its resistance changes depending on a temperature. For this reason, in the case of constant-voltage driving, the current i flowing through the copper wire coil changes depending on a temperature, and hence, a drive force is changed. The above-mentioned electro-magnetic transducer system generally has a motional impedance, and the resistance component of the copper wire coil serves as a damping resistance of this motional impedance. Therefore, a damping force also changes in accordance with a temperature.
In order to obtain a larger drive force and damping force than those of the conventional constant-voltage driving, there is proposed a system in which a negative impedance is equivalently generated in a driver side, and a load is negative-impedance driven through the negative impedance. In order to equivalently generate the negative impedance, a current flowing through the load must be detected. For this purpose, a detection element is connected in series with the load. In the system of performing negative-impedance driving, since an impedance of the load is apparently canceled by the equivalently generated negative impedance, a large drive force and damping force can be simultaneously realized. FIG. 4 is an equivalent circuit diagram of this system. In FIG. 4, reference symbols C.sub.M and L.sub.M denote a capacitance component and an inductance component of a motional impedance Z.sub.M of an electro-magnetic transducer (speaker), respectively; and R.sub.V, an internal resistance of a voice coil as a load. The internal resistance R.sub.V is eliminated by a negative resistance -R.sub.O equivalently formed at the driving side, and an apparent drive impedance Z.sub.A is given by: EQU Z.sub.A =R.sub.V -R.sub.O
If Z.sub.A becomes negative, the circuit operation is rendered unstable. Therefore, the values of R.sub.V and R.sub.O are set as R.sub.V .gtoreq.R.sub.O.
In the conventional negative-impedance driving system, the large drive force and damping force are realized. However, it is difficult to obtain a uniform drive impedance with respect to a motional impedance over a wide temperature range. For example, in the circuit of FIG. 4, if the equivalent negative resistance -R.sub.O is set to be constant regardless of a temperature, the ratio of influence of a change in resistance of R.sub.V caused by a change in temperature with respect to the drive impedance Z.sub.A becomes larger than that in the case of the constant-voltage driving.
In a driving apparatus disclosed in U.S. patent application Ser. No. 07/357,701 assigned to the present assignee, a temperature coefficient of a detection element for detecting a current flowing through a load is set to be equal to or slightly larger than a load impedance of the load, thereby eliminating the conventional drawbacks.
In the negative impedance driving apparatus of the application U.S. Ser. No. 07/357,701, when the load impedance is increased according to an increase in temperature of the load, the absolute value of the negative impedance is also increased. In view of an actual amplifier output, the negative impedance driving is to supply a drive voltage higher than that in a normal drive mode, i.e., a drive power to the load as a negative impedance component, and hence, load power consumption is increased as the negative impedance has a larger absolute value. The power supplied to the load basically causes heat dissipation. For this reason, assuming that a load temperature is increased and the load impedance is increased while an input signal is constant, the driving apparatus having the circuit of the prior application may thermally run away such that the negative impedance is temperature-compensated, its absolute value is increased, power supplied to the load is increased, the load temperature is further increased, load impedance and the negative impedance are further increased, and so on. Thus, the driving apparatus of the application U.S. Ser. No. 07/357,701 has no thermal protection means.
Assuming a maximum output state as a thermal design condition of the driving apparatus and the load, the driving apparatus and the load can rarely be thermally destroyed. Assuming the maximum output taking into consideration that the negative-impedance driving is performed and a drive condition changes due to temperature compensation, the maximum output becomes a considerably larger value than in a normal design condition. Therefore, a problem of excessively high quality may be derived with reference to a conventional thermal stability decision standard.